Solder compositions such as a so-called solder paste formed from a mixture of a solder alloy powder and a flux, and a so-called resin flux cored solder in which a flux is filled inside a wire-shaped solder alloy, for example, are used for mounting electronic components on an electronic circuit substrate, such as a printed circuit substrate. As the flux used in such solders, generally resin fluxes including a synthetic resin, a rosin resin, or the like as a resin component are widely used.
The above-described flux is blended in a solder composition and the like in order to improve solderability by removing metal oxides on a surface conductive portion of the printed circuit substrate, preventing the solder alloy from reoxidizing during soldering, or reducing the surface tension of the solder.
On the other hand, when soldering using a solder that uses the above-described flux, a film portion referred to as flux residue resulting from the resin component in the flux remains on the electronic circuit mounted substrate after the soldering. This flux residue can contain fissures (cracks) caused by temperature changes during the thermal cycle and the like. If moisture seeps into the flux residue from the cracks, this can cause defects such as a deterioration in the insulation between leads.
Especially, in an electronic circuit mounted substrate that is used in an area subjected to large temperature differences, such as an electronic circuit mounted substrate used in the engine of an automobile and the like, cracks tend to occur in the above-described flux residue, and the occurrence of these cracks needs to be suppressed.
Various technologies exist for suppressing such cracks.
For example, Patent Literature 1 and Patent Literature 2 describe a flux that includes an acrylic resin having a low glass transition temperature.
Patent Literature 3 describes a flux that includes hydrogenated 1,2-polybutadiene and the like as a resin component.
Patent Literature 4 describes a flux that includes a polyamide resin having a softening point of 80 to 150° C. that is obtained by a condensation reaction of a dimer acid and a diamine.
Patent Literature 5 describes a flux that, in addition to including rosin and an activator, also includes an ethylene vinyl acetate copolymer.
Patent Literature 6 describes a flux that includes as a resin component a compound in which acrylic groups or methacrylic groups are bonded to the molecular end of atactic 1,2-polybutadiene or a hydride thereof.
However, the fluxes described in Patent Literatures 1 and 2 suffer from the following problem: the flux residue is sticky and contamination from dust and the like tends to occur. The fluxes described in Patent Literatures 3 to 6 suffer from the following problem: although a certain level of solder wettability can be obtained under low-temperature preheating conditions of 160° C. or less, under the high-temperature preheating conditions of more than 160° C. that are required for lead-free solder and the like, sufficient solder wettability is not obtained, and dewetting can occur.
In addition, the fluxes described in Patent Literatures 1 to 6 all suffer from the following problem: since solder wettability cannot be sufficiently exhibited under reflow heating in an air atmosphere, only reflow in a nitrogen atmosphere can be carried out.